802.1ag | 87Continuity Check MessagesContinuity Check Messages (CCM) are periodic hellos used to:• discover MEPs and MIPs within a maintenance domain• detect loss of connectivity between MEPs• detect misconfiguration, such as VLAN ID mismatch between MEPs• to detect unauthorized MEPs in a maintenance domainContinuity Check Messages (CCM) are multicast Ethernet frames sent at regular intervals from each MEP.They have a destination address based on the MD level (01:80:C2:00:00:3X where X is the MD level ofthe transmitting MEP from 0 to 7). All MEPs must listen to these multicast MAC addresses and processthese messages. MIPs may optionally processes the CCM messages originated by MEPs and construct aMIP CCM database.MEPs and MIPs filter CCMs from higher and lower domain levels as described in Table 5-1.All the remote MEPs in the maintenance domain are defined on each MEP. Each MEP then expects aperiodic CCM from the configured list of MEPs. A connectivity failure is then defined as:1. Loss of 3 consecutive CCMs from any of the remote MEP, which indicates a network failure2. Reception of a CCM with an incorrect CCM transmission interval, which indicates a configurationerror.3. Reception of CCM with an incorrect MEP ID or MAID, which indicates a configuration orcross-connect error. This could happen when different VLANs are cross-connected due to aconfiguration error.4. Reception of a CCM with an MD level lower than that of the receiving MEP, which indicates aconfiguration or cross-connect error.5. Reception of a CCM containing a port status/interface status TLV, which indicates a failed bridge oraggregated port.The Continuity Check protocol sends fault notifications (Syslogs, and SNMP traps if enabled) wheneverany of the above errors are encountered.Table 5-1. Continuity Check Message ProcessingFrames at Frames from UP-MEP Action Down-MEP Action MIP ActionLess than my level Bridge-relay side or Wire side Drop Drop DropMy level Bridge-relay side Consume Drop Add to MIP-DBand forwardWire side Drop ConsumeGreater than my level Bridge-relay side or Wire side Forward Forward Forward
